A semiconductor device including a semiconductor package and a cooler for cooling the semiconductor package has been proposed. The semiconductor package includes a semiconductor element and a metal member. The semiconductor element and the metal member are covered with a molding member to form the semiconductor package. A mounting surface of the metal member is connected to the semiconductor element so that heat of the semiconductor element can be transmitted to the metal member. A radiation surface of the metal member is exposed outside the molding member.
The cooler is attached to the semiconductor package in such a manner that the radiation surface of the metal member is in contact with the cooler through an electrically insulating grease. Thus, the heat of the semiconductor package is released to the cooler.
The cooler is typically made of metal. Therefore, as disclosed in, for example, JP-A-2008-166333 or US 2004/0089928 corresponding to JP-3740116, the radiation surface of the metal member is covered with an electrically insulating layer to prevent a short-circuit between the metal member and the cooler. Further, the insulating later is covered with an electrically conducting layer for protecting the insulating layer. The conducting layer of the metal member is in contact with the cooler through the grease.
That is, the metal member has a multilayer structure including a metal portion, an electrically insulating layer on the metal portion, and an electrically conducting layer on the insulating layer. The conducting layer serves as the radiation surface of the metal member.
The present inventors have found out that such a semiconductor device has the following disadvantages. The disadvantages are discussed below with reference to FIGS. 15 and 16.
FIG. 15 is a cross-sectional view of a first conventional semiconductor device, and a FIG. 16 is a cross-sectional view of a second conventional semiconductor device.
Firstly, the second conventional semiconductor device shown in FIG. 16 is discussed. In the second conventional semiconductor device, a heat radiation plate 31 is mounted on a cooler 200 through a grease 300 having a heat conductivity. An electrically insulating substrate 33 is mounded on the radiation plate 31 through a solder 32. A semiconductor element 10 is mounted on the insulating substrate 33 through the solder 32.
The radiation plate 31 is fixed to the cooler 200 by a bolt 34 that reaches the radiation plate 31 by penetrating the radiation plate 31 and the grease 300.
Thus, the radiation plate 31 is pressed by the bolt 34 against the cooler 200 by a pressure necessary to allow the heat to be transmitted from the radiation plate 31 to the cooler 200.
In this case, since the insulating substrate 33 is located directly under the semiconductor element 10, heat of the semiconductor element 10 is transmitted to the insulating substrate J3 at a high heat flux. As a result, an increase in temperature of the insulating substrate J3 is large.
Secondly, the first conventional semiconductor device shown in FIG. 15 is discussed.
The first conventional semiconductor device includes a semiconductor package 100 having a semiconductor element 10 and a metal member 20. The semiconductor element 10 and the metal member 10 are covered with a molding member 60 to form the semiconductor package 100. A mounting surface of the metal member 20 is connected to the semiconductor element 10. A radiation surface of the metal member 20 is exposed outside the molding member 60. The metal member includes a metal portion 21, an electrically insulating layer 22 on the metal portion 21, and an electrically conducting layer 23 on the insulating layer 22. The conducting layer 23 serves as the radiation surface of the metal member 20.
The first conventional semiconductor device further includes a cooler 200 having a coolant passage 201 through which a coolant 202 circulates. The conducting layer 23 of the semiconductor package 100 is in contact with the cooler 200 through an electrically insulating grease 300. Thus, the heat of the semiconductor package 100 is absorbed by the coolant 202 of the cooler 200 so that the conducting layer 23 can be cooled by the coolant 202.
That is, in the first conventional semiconductor device, the semiconductor element 10 is located through the metal portion 21 on the insulating layer 22. Thus, heat of the semiconductor element 10 is radiated to the metal portion 21 and thus transmitted to the insulating layer 22 at a low heat flux. Therefore, the first conventional semiconductor device can have a high heat radiation performance compared to the second conventional semiconductor device. Accordingly, the first conventional semiconductor device can be reduced in size.
By the way, in the first conventional semiconductor device, when a voltage V0 caused by a switching operation of the semiconductor element 10 is applied to a mounding surface (i.e., metal portion 21) of the metal member 20, a voltage V2 is applied to the radiation surface (i.e., conducting layer 23) of the metal member 20.
Specifically, the voltage V2 is given by the following equation:V2={C1/(C1+C2)}·V0
In the above equation, C1 represents a capacitance of a parasitic capacitor formed between the metal portion 21 and the conducting layer 23 through the insulating layer 22. C2 represents a capacitance of a parasitic capacitor formed between the cooler 200 and the conducting layer 23 through the grease 300. When the voltage V2 is applied to the conducting layer 23, noise radiation or partial discharge from the conducting layer 23 may occur. Such noise radiation may damage the grease 300, for example.
The application of voltage V2 to the conducting layer 23 can be prevented by electrically connecting the conducting layer 23 to the cooler 200 through the grease 300. However, since the grease 300 has an electrically insulating property, it is difficult to ensure electrical connection between the conducting layer 23 and the cooler 200. Further, it is difficult to check the electrical connection between the conducting layer 23 and the cooler 200.